1. Field of the Invention
This invention relates to apparatus for chemical vapor deposition (CVD) of coatings onto substrates such as flat glass substrates, and, more particularly, to apparatus suitable for forming metal-containing and like films having advantageous properties in an efficient and economical manner.
2. Description of the Prior Art
The desirability of applying uniform coatings to a flat glass substrate for the purpose of varying its thermal, optical and/or electrical properties has long been recognized. Such coatings generally consist of a metal or metal oxide, particularly tin oxide. In such process, a hot, freshly formed glass ribbon travels from a flat-glass forming section to an annealing section, where the coating is deposited on at least one face thereof. The coating applicator generally consists of one or more nozzles which direct a spray of coating-chemical reactant in a carrier gas onto the exposed face of the glass ribbon. Thereafter, reaction by-products and unused coating compound are removed by exhaust means.
Conventional coating apparatus, systems and processes are disclosed, inter alia, in U.S. Pat. Nos. 4,598,023; and 4,601,917; in EPO No. 0188962; an article entitled "Production of Chemical Vapor Deposition Coatings" by Nicholas M. Gralenshi, Watkins-Johnson Co., Scotts Valley, Calif. 95066, pages 102-144; and an article entitled "Coating of Glass by Chemical Vapor Deposition" by John M. Blocher, Jr. in Thin Solid Films, No. 77, pages 51-63 (1981).
In the CVD process, it is desirable to apply to flat glass substrates coatings which are uniform, haze-free, have a low electrical resistivity, and which are formed during a relatively short deposition time.
Although each of these requirements can be met individually, it is exceedingly difficult to satisfy all of them at the same time. For example, high substrate temperatures will provide short deposition times, and coating chemicals containing a dopant precursor, applied at high substrate temperatures, result in films having lower electrical resistance. Moreover, high deposition temperatures favor increased haze in the coatings formed. Similarly, haze-free films can be achieved by using a low surface temperature and relatively low water-vapor content in the coating gases, albeit at the expense of reduced deposition rate and poorer electrical properties. Uniform coatings also require the application of a substantially uniform coating-chemical composition at uniform vapor velocities on each spot on the surface of the glass substance, a condition difficult to achieve with known nozzle applications. In short, conventional CVD systems are found to be deficient with respect to one or more of these film qualities and process parameters.
Furthermore, with prior-art systems, a larger exhaust for removing spent gases has generally been required. Such large exhausts tend to draw outside gases, e.g. air, into the coating zone, which action dilutes the concentration of coating vapors. On the other hand, a small exhaust system will allow coating gases to escape from the coating zone and become admixed with the outside atmosphere. In both cases, the diluted vapors which contact the glass substrate produce unacceptably hazy films. Moreover, large amounts of exhaust reduce the efficiency of use of coating material, and increase the cost of recovery of coating material from the exhaust gases.
Many of these prior CVD systems use nozzles positioned adjacent to the surface of the substrate, and which are provided with clearances between the nozzle and substrate and between the exhaust port and substrate. These open systems result in the introduction and mixing of outside air with coating vapors, causing haze, as shown, for example, in U.S. Pat. No. 4,123,244.
Further, with conventional CVD systems, the assumed relationships between coating-gas velocity, chemical concentration and chemical consumption, with nozzle slit width, have provided coating applicators which are less than satisfactory. Specifically, such relationships have required a nozzle applicator with a rather narrow slit width, which causes extreme problems with film uniformity and nozzle clogging. In addition, such systems consume a large amount of chemical reactant, resulting in an uneconomical process. On the other hand, systems which use a low gas velocity or low chemical concentration tend to effect deposition at a rate which is controlled primarily by the diffusion of chemicals to and from the substrate surface; this tends to produce a coating which is hazy and has a rough surface.